The answer to this depends on such a variety of factors that it's impossible to give a useful answer. Use something like New Relic to watch your app's performa

The answer to this depends on such a variety of factors that it's impossible to give a useful answer. Use something like New Relic to watch your app's performance. If it's exceeding tolerable limits, you need to change something. This might be a re-write of your code to reduce load, or pouring more resource into dynos, workers, database or caching.